•iOO STUDIES IN EVOLUTION 



variations from the typical form is not of much moment in 

 tliis place, although the geologic history and interpretation 

 of these differences are of considerable interest. Two well- 

 defined varieties or species are recognized in Sweden, and are 

 represented in outline by figures 2 and 28, Plate XXIII. 

 The prevailing form in the Wenlock shales at Dudley, Eng- 

 land, agrees with figure 28, and also represents the ordinary 

 form from the Niagara Group of Indiana and New York. 

 Each locality, however, presents minor differences, mainly 

 of local interest, and seldom of varietal importance. In 

 western New York, besides the ordinary form with both 

 valves convex, there is found an arcuate, deeply bilobed 

 variety, agreeing with the extreme of the Swedish B. hilohus, 

 var. Ve?-neiiilianus Lindstrom, represented in Plate XXIII, 

 figure 2. The lobes of the New York variety are commonly 

 more divergent, as shown in the outline, Plate XXIII, 

 figure 1. This form wa,s recently described by Ringueberg 

 as Orthis acutiloha.* 



The Lower Helderberg species known as B. various Conrad, 

 sp., presents an amount of departure from typical B. bilobus, 

 as would be anticipated from the change in the chronological 

 and physical conditions of the species, combined with its 

 extremely prolific development at this time. The abundance 

 and comparatively large size of individuals clearly indicate 

 most favorable conditions for their existence and multiplica- 

 tion, and, also, for the assumption and transmission of any 

 varietal forms in harmony with the environment. 



Mature individuals from Dudley, England, and Gotland, 

 Sweden, represented by figure 28, Plate XXIII, correspond 

 in all characters with specimens of B. various which are 

 about half or two-thirds grown. After reaching the adult 

 bilobus stage, B. various continues its growth, but this sub- 

 sequent increment is gerontic in its nature, although such 

 senile features are here the conditions of simple maturity or 

 the completed ephebic stage. Evidences of this are seen in 

 the gradual obsolescence of the pronounced lobation of the 



* Proc. Acad. Nat. Sci. Phila., 134, 1888. 



